Photomechanical Effects in Azo-Polymers Studied by Neutron Reflectometry


Yager, K.G.; Tanchak, O.M.; Godbout, C.; Fritzsche, H.; Barrett, C.J. "Photomechanical Effects in Azo-Polymers Studied by Neutron Reflectometry" Macromolecules 2006, 39 9311.
doi: 10.1021/ma0617320


Using neutron reflectometry, we were able to unambiguously show that azo-polymer materials photo-expand under typical conditions. Moreover, we have identified a unique form of photo-contraction that occurs when the sample is heated above a critical temperature. These photo-motions provide insight into the fundamental nature of azo mobility.


Neutron reflectometry is used to study photomechanical effects in thin films of azobenzene polymer cast onto silicon substrates. A significant photoexpansion effect, up to 17%, is observed at 25 C, due to the free volume requirement of the azobenzene chromophore photoisomerization. Above a distinct crossover temperature of ~50 C, the material response is inverted and instead photocontraction effects, of more than -15%, are observed. In this case the combined photomotion and thermal mobility enables aggregation and crystallization of the azobenzene dipoles. The photomechanical effects, which can be reversed, occur readily using a variety of irradiation powers, incident polarizations, and film thickness values. This photomechanical behavior, which appears to be general to all azo materials, is likely the origin for a wide variety of curious photomotions observed in these systems, including macroscopic bending of samples and micron-scale surface mass transport.